Modelling the global atmospheric ammonia budget

Anthropogenic ammonia emissions are increasing rapidly in many areas of the world, and are expected to continue to do so due to increasing food production and warmer temperatures leading to higher rates of ammonia emissions. Ammonia is a critical species in the nitrogen cycle as it volatilises from fertilizer and manure, and it is also the principal alkaline gas in the atmosphere. It therefore plays an important role in atmospheric chemistry, reacting with sulfuric and nitric acids to form ammonium aerosols. The presence of excess ammonia or ammonium reduces soil, water and air quality, negatively affecting human and ecosystem health. In this work, the fate of reduced nitrogen is studied using global and regional ammonia and ammonium nitrate budgets simulated by the chemistry-climate model UKCA coupled to the Unified Model, with the aerosol scheme CLASSIC. The aims of the thesis are: to evaluate the UKCA-CLASSIC ammonia distribution by comparing model results with observations; to present details of global and regional ammonia and ammonium nitrate fluxes and lifetimes, quantifying the important processes that control reduced nitrogen in the atmosphere and to discuss regional and seasonal differences in the atmospheric budgets; and to model the impacts of changes in temperature and ocean acidification on the flux of NH3 between the ocean and the atmosphere using a new interactive scheme for ocean-atmosphere NH3 exchange. First, global atmospheric ammonia distributions are compared with ground-based observations, vertical profile measurements, and satellite retrievals. Model concentrations match surface observations well in Europe, North America, and Japan. The model underestimates concentrations over India and Africa. The global mean bias is -0.4 μg/m3 (model - observations), indicating the model captures the overall surface ammonia distributions reasonably well, although this value is heavily influenced by the uneven distribution of measurement locations. The modelled vertical profile of ammonia compares well with observations. The comparison of the model results with satellite retrievals finds similar overall global patterns. Model concentrations at 918 hPa are on average 1.0 to 1.5 ppbv larger than those reported by the Atmospheric Infrared Sounder (AIRS), depending on the season. These results suggest the model ammonia distributions are informative and useful, but also indicate some improvements could be made in emission inventories or model deposition or reaction fluxes. This model is then used to provide global distributions, deposition fluxes and lifetimes of ammonia and ammonium nitrate, quantifying the important processes that control reduced nitrogen in the atmosphere. Sinks for ammonia and ammonium nitrate are strongly influenced by weather as precipitation patterns drive wet deposition. Weather and climate therefore affect the atmospheric lifetimes of ammonia and ammonium nitrate and this contributes to regional and temporal differences in air quality. Model results show the ammonia lifetime differs across regions; it is shortest in Europe (0.5 days), followed by East Asia (0.7 days), and North America (1.0 day), while ammonia in South Asia has a relatively longer lifetime of 1.4 days (nearly three times that of Europe). The largest sink for ammonia globally is wet deposition (11.9 Tg N/yr), followed by dry deposition (10.9 Tg N/yr). The formation of ammonium sulfate aerosol consumes 10.3 Tg N/yr of ammonia. The reaction to form ammonium nitrate is reversible and the net quantity of ammonia consumed is 9.7 Tg N/yr. Although on a global scale these numbers are comparable, the relative influence of the sinks differs substantially by region. The dominant sink over the largest surface area is ammonium sulfate formation over the ocean. In areas with high ammonia emissions, the largest sinks are typically dry deposition and ammonium nitrate formation. Finally, the effects of temperature and ocean pH are explored on the ocean-atmosphere exchange of ammonia. An interactive scheme for this exchange was developed and implemented in the model. The scheme takes into account future projections of changes in temperature and ocean pH. Results show that ocean acidification has the largest effect, leading to a decrease in global ocean ammonia emissions from a range of 1.6 to 3.1 Tg-N/yr for the present day to a range of 0.56 to 1.7 Tg-N/yr for 2100 (RCP 8.5), reducing the oceanic source of ammonia to the atmosphere and suggesting this is one of several routes through which the flux of nitrogen to the oceans will increase in the future.